Photopolymerizable compositions fundamentally comprise a photopolymerization initiator and a polyfunctional monomer, and are hardened upon being irradiated with light to become insoluble in solvents. Such compositions are widely used in photography, printing, surface processing of metals, inks, etc., utilizing the above-noted property. See J. Kosar, Light Sensitive Systems, J. Wiley & Sons., New York, 1965, pp. 158-193.
Studies on photopolymerizable compositions have been made to enhance sensitivity thereof to light, and many photopolymerization initiators have been proposed; for example, benzoin ethers are described in U.S. Pat. No. 2,448,828, benzoins are described in U.S. Pat. No. 2,722,512, anthraquinones are described in U.S. Pat. No. 3,046,127, aminophenylketones and active methyl or amino compounds are described in Japanese Patent Publication No. 11936/74 (corresponding to U.S. Pat. No. 3,661,588), Michler's ketone and benzophenone are described in U.S. Pat. No. 3,682,641, benzophenone and 4-N,N-dimethylaminobenzophenone are described in Japanese Patent Publication No. 38403/73 (corresponding to U.S. Pat. No. 3,549,367), etc. These photopolymerization initiators generally improve photosensitivity to some extent, but the resulting photo-hardened products (particularly film-like hardened products) have mechanical properties that are not necessarily sufficient for various end-uses.
For example, when used as dry film resist to be used in preparing printed circuit plates, they provide insufficient film strength after photo-hardening. Japanese Patent Publication No. 25231/70 (corresponding to U.S. Pat. No. 3,469,932) describes dry film resists for preparing printed circuit plates, and more detailed descriptions are set forth, for example, in W. S. De Forest, Photo-resist, McGraw-Hill, New York, 1975, pp. 163-212. The main use of dry film resist is for preparation of through-holes by tenting. However, conventionally known photopolymerization initiators have often caused the problem of film breakage during developing and etching steps due to insufficient strength of the tenting film (hereinafter, referred to as "tent").
On the other hand, in some cases exposed portions and unexposed portions are required to be discriminated from each other in exposing procedures. For example, when procedures are interrupted in the course of exposing many presensitized printing plates at the same time, plate-making workers want to known whether plates given to them have been exposed or not. In the case of exposing a large plate many times, there is a need to know to what extent the plate has been exposed. In such cases, it is preferable that exposed portions and unexposed portions can be discriminated from each other. For this purpose, a photopolymerizable composition including a free radical-generating agent capable of generating a free radical upon irradiation with light and an agent which undergoes a change in color by reaction with the free radical has been employed, to thereby form a visible image after exposure. As the free radical-generating agent, organic halogen compounds are useful, including carbon tetrabromide, iodoform, and phenyltribromomethylsulfone as disclosed in Japanese Patent Publication No. 29407/68 (corresponding to U.S. Pat. No. 3,502,476), 2-halomethyl-5-vinyl-1,3,4-oxadiazole compounds as disclosed in Japanese Patent Application (OPI) No. 24113/80 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".), etc.
However, the goal of obtaining a high-contrast visible image after long-term storage without detrimental influences on film strength of photo-hardened film has not been fully attained.